ci: Upload artifacts on failure without specifying a directory

on_failure allows to retrieve artifacts in case of failure, and only in that case: see https://docs.gitlab.com/ee/ci/yaml/index.html#artifactswhen

The build takes place in a directory of the form xfconf-${version}git${commit}/_build/sub at the moment, but the _build/sub part could possibly change in the future, as it is added by Gitlab. So the easiest and most robust way seems to be to recover the test-suite.log files without worrying about the path.

See also !19 (comment 43611)